text: Because the previous synthesis had worked, photo-methionine is photo-reactive due to the diazirine ring. Once this ring has become exposed to UV light, nitrogen leaves as nitrogen gas (N) and forms the highly reactive intermediate carbene. Photo-activation of amino acids provide the ability of photo-cross-linking in proteins. This type of cross-linking has three major advantages; there is greater specificity for this cross linking due to the short lived intermediates and that this amino acid is functional, and most importantly; not toxic (meaning it should not disrupt the protein's function or structure dramatically). Research had found that this activation is the rate-limiting step; not the cross-linkage.